1. Field of the Invention
This invention relates to directed energy weapons, and more particularly, to high power microwave weapons.
2. Brief Description of Related Art
Conventional high power microwave (HPM) sources incorporating pulse forming networks (PFNs) based on segments of charged transmission lines (also known as a family of Darlington circuits) have fixed spectral content which is determined by the electrical length. However, potential customers indicate a lot of interest in frequency agile HPM sources and in sources capable of generating arbitrary waveforms since different targets are vulnerable to different frequencies and different waveforms.
As is conventional in the art and used within this patent application, a pulse forming network (PFN) is an electric circuit that accumulates electrical energy over a comparatively long time, then releases this stored energy in the form of a relatively square pulse of a comparatively brief duration for various pulsed power applications. In the PFN circuit, the energy storage components, such as capacitors, inductors or transmission lines are charged by means of a high voltage power source, then rapidly discharged into a load via a high voltage switch, e.g., as a spark gap, a hydrogen thyratron or a photoconductive semiconductive switch. Repetition rates range from single pulses to about 104 per second. PFN circuits are used to produce precise nanosecond-length pulses of electricity to power devices such as klystron or magnetron tube oscillators in radar sets, pulsed lasers, particle accelerators, flashtubes, and high voltage utility test equipment. A lot of high energy research equipment is operated in a pulsed mode, both to keep heat dissipation down and because high energy physics often occur at short time scales, so large PFN circuits are widely used in high energy research. They have been used to produce nanosecond length pulses with voltages of up to 106-107 volts and currents up to 106 amps, with peak power in the terawatt range, similar to lightning bolts.
Conventional PFN circuits consist of the segments of transmission lines having the same electrical length but different impedances. An example of such pulse forming circuits is the Blumlein pulse-forming line. The Blumlein pulse-forming line is, in fact, the simplest member of the entire family of circuits known as the Darlington circuits. These circuits are capable of producing unipolar (Darlington) or bipolar (S. London) single-cycle rectangular pulses as well as trains of such pulses. These circuits also produce such pulses at potentials which can be many multiples of the potential to which the circuit is charged initially. The electrical length of the transmission line segments in conventional pulse-forming networks is fixed. This fixed electrical length, in turn, fixes duration of the generated pulses as well as their spectral content.
With reference now to FIG. 1, a description concerning a conventional prior art circuit 2 will now be briefly described. As is conventional in the art, the central core component of such prior art circuit 2 is a block of dielectric material 4, e.g., polypropylene, ceramics, etc. The dielectric material 4 is then cut or otherwise processed in order to remove undesired dielectric material 4 and thereby form the basic shape of the circuit 2, i.e., both a generally linear surface 6, along one elongate side of the dielectric material 4, and a generally stepped shaped surface 8, along an opposite elongate side of the dielectric material 4, with the dielectric material 4 located therebetween. As shown in this Figure, the generally stepped shaped surface 8 is formed to produce a series of at least seven steps or stages 10 with each step or stage 10 being located progressively further away from the associated generally linear surface 6. For example, the first step or stage 10′ may be spaced from the generally linear surface 6 by a distance of a few thousands of an inch or so, the second step or stage 10″ may be spaced from the generally linear surface 6 by a distance of 15 to 20 thousands of an inch or so, the third or stage 10′″ may be spaced from the generally linear surface 6 by a distance of 60 to 80 thousands of an inch or so, and so forth.
Next, both the generally linear surface 6 and the generally stepped shaped surface 8 are each covered with a thin layer of a copper material 12 (e.g., by an electroplating process for example) thereby to form a generally linear ground section 14 of the circuit 2 and also form a generally stepped shaped charged section 16 of the circuit 2. Each step or stage, of the generally stepped shaped charged section 16 of the circuit 2, typically has a thickness of between 0.003 and 0.008 of an inch, a width of between 2.75 inches (7 cm) and 3.15 inches (8 cm) and an axial length—depending upon the frequency—of between 0.39 inches (1 cm) and 15.75 inches (40 cm), for example. The ground section 14 of the circuit 2, on the other hand, typically has a thickness of between 0.003 and 0.008 of an inch, a width of between 2.75 inches (7 cm) and 3.15 inches (8 cm) and an axial length, between 2.75 inches (7 cm) and 110.23 inches (280 cm) or more, for example. That is, the axial length of the ground section 14 of the circuit 2 is equal to a total combined axial length of each of the steps or stages.
As shown in FIG. 1, a final step or stage Z7′ of the generally stepped shaped charged section 16 of the circuit 2 and is spaced or otherwise separated from the last step or stage of the generally stepped shaped charged section 16, e.g., the seventh step or stage Z7 in this circuit 2, by a layer of unremoved dielectric material 4′. The final step or stage Z7′ typically has a thickness of between 0.003 and 0.008 of an inch, a width of 7 or 8 centimeters and an axial length, typically between 0.39 inches (1 cm) and 15.75 inches (40 cm), for example. As shown in FIG. 1, a left end beginning of the last step or stage Z7′ of the charged section 16 of the circuit 2 is axially aligned with a left first end of the final step or stage Z7 while the axial length of the final step or stage Z7′ is axially longer than the axial length of the last step or stage Z7 of the charged section 16 of the circuit 2 so that the right end of the final step or stage Z7′ terminates at the same axial position as the ground section 14 of the circuit 2 terminates.
As is conventional in the art, a switch 18 is coupled to a first (e.g., left end) axial end of both the ground section 14 and the charged section 16 to facilitate periodic discharge of the circuit 2. This switch 18 may be periodically activated by a laser (not shown), for example, for discharging the circuit 2, as desired. The circuit 2 is also typically charged, via a conventional power supply 19, to a voltage of between 1,000 and 7,000 volts, for example, before being periodically discharged by the switch 18, e.g., a laser being fired at the switch 18 in order to activate the switch 18 and discharge the circuit 2. In a conventional manner, a first contact of the power supply 19 is coupled to the copper material 12 of the ground section 14 while a second contact of the power supply 19 is coupled to the copper material 12 of the charged section 16. In order to complete the circuit 2, a first end of an antenna ZL is coupled to an opposite second (right) axial end of the ground section 14 while a second end of the antenna ZL is coupled to an opposite (right) second end of the final step or stage Z7′ to facilitate periodic propagation of the high power microwave energy from the circuit to the antenna ZL and into the surrounding environment. As is conventional in the art, when the circuit 2 is discharged, this causes a high power microwave signal to be generated by the first step or stage Z1. Thereafter, this generated high power microwave signal propagates progressively along the circuit 2, from left to right, and eventually to the antenna ZL for transmission, e.g., from the first step or stage Z1 to the second step or stage Z2, from the second step or stage Z2 to the third step or stage Z3, from the third step or stage Z3 to the fourth step or stage Z4, from the fourth step or stage Z4 to the fifth step or stage Z5, from the fifth step or stage Z5 to the six step or stage Z6, from the sixth step or stage Z6 to the seven step or stage Z7, from the seven step or stage Z7 to the last step or stage Z7′, and from the last step or stage Z7′ into the antenna ZL for transmission. The antenna ZL then propagates the high-power microwave signal into the surrounding environment and typically toward a desired target 22.
While the above arrangement has worked satisfactory for some applications, there still exists a need for a way to vary the waveform and its spectral content in order to provide a more versatile high-power microwave signal for achieving the desired result.